The fluxes between 2-deoxyglucose and 2-deoxyglucose phosphate in brain were examined in rats in vivo using tracer kinetic techniques. Rate constants f the fluxes can be derived from two separate experiments with different routes of tracer administration. In one experiment, tracer 2-deoxyglucose was admininistered through the internal carotid artery. In the other experiment, tracer 2-deoxyglucose was administered through the atrium. The rate constant for the forward reaction from 2-deoxyglucose to 2-deoxyglucose phosphate can be calculated by dividing the initial rate of 2-deoxyglucose phosphate formation by the initial content of 2-deoxyglucose in brain. The rate constant for the reverse reaction from 2-deoxyglucose phosphate to 2-deoxyglucose can be calculated from the decay constant of 2 deoxyglucose phosphate, starting at a time when tracer 2-deoxyglucose derived from the initial injection has been exhausted. The rate constants calculated were 15.4%/min and 4.1%/min respectively for the forward and reverse reactions. Rate constants for both reactions can be estimated from the data derived from the intravenous experiments with aid of a computer. The rate constants for the forward and reverse reactions were estimated as 11.4%/min and 5.1%/min respectively by fitting the data to program SAAM-27. The suitability for using 2-deoxyglucose to trace the rate of glucose utilization in brain were discussed by comparing the rate constants derived from 2-deoxyglucose and tracer glucose. It was concluded that the rate measured from 2-deoxyglucose is not the true rate of glucose utilization. It only reflects the reaction at hexokinase in brain.